This invention relates generally to color cathode ray tubes (CRTS) and is particularly directed to the control of multiple electron beams incident upon the faceplate of a color CRT.
Most color CRTs employ an inline electron gun arrangement for directing a plurality of electron beams on the phosphorescing inner screen of its glass faceplate. The inline electron gun approach offers various advantages over earlier "delta" electron gun arrangements particularly in simplifying the electron beam positioning control system as well as essentially eliminating the tendency of the electron beams to drift. However, inline color CRT's employ a self-converging deflection yoke which applies a nonuniform magnetic field to the electron beams, resulting in an undesirable astigmatism in and defocusing of the electron beam spot displayed on the CRT's faceplate. In order to achieve three electron beam convergence at the screen edges and corners, the self-converging yoke applies a dynamic quadrupole magnetic field to the beams which over-focuses the beams in the vertical direction and under-focus them in the horizontal direction. This is an inherent operating characteristic of the inline yoke design.
One approach to eliminate this astigmatism and deflection defocus employs a quadrupole lens with the CRT's focusing electrode which is oriented 90.degree. from the sel-fconverging yoke's quadrupole field. A dynamic voltage, synchronized with electron beam deflection, is applied to the quadrupole lens to compensate for the astigmatism caused by the deflection system. This dynamic voltage also allows for dynamic focusing of the electron beams over the entire CRT screen. The astigmatism of the electron beam caused by the quadrupole lens tends to offset the astigmatism caused by the color CRT's self-converging deflection yoke and generally improves the performance of the CRT.
An article entitled "Progressive-Scanned 33-in. 110.degree. Flat-Square Color CRT" by Suzuki et al published in SID 87 Digest, at page 166, discloses a dynamic astigmatism and focus (DAF) gun wherein spot astigmatism and deflection defocusing are simultaneously corrected using a single dynamic voltage. The electron gun employs a quadrupole lens to which the dynamic voltage is applied and which includes a plurality of generally vertically elongated apertures in a first section of a focusing electrode and a second pair of aligned, generally horizontally oriented elongated apertures in a second section of the focusing electrode. Each electron beam first transits a vertically aligned aperture, followed by passage through a generally horizontally aligned aperture in the single quadrupole lens for applying astigmatism correction to the electron beam.
An article entitled "Quadrupole Lens For Dynamic Focus and Astigmatism Control in an Eliptical Aperture Lens Gun" by Shirai et al, also published in SID 87 Digest, at page 162, discloses a quadrupole lens arrangement comprised of three closely spaced electrodes, where the center electrode is provided with a plurality of keyhole apertures and the outer electrodes are provided with a plurality of square recesses each with a circular aperture in alignment with each of the respective electron beams. A dynamic voltage V.sub.d is applied to the first and third electrodes so as to form a quadrupole field to compensate for the astigmatism caused by the self converging yoke deflection system. Although this allows for a reduction in the dynamic voltage applied to the quadrupole, this voltage still exceeds 1 KV in this approach. While these two articles describe improved approaches for beam focusing and astigmatism compensation, they too suffer from performance limitations particularly in the case of those CRTs having a flat faceplate and foil tension shadow mask, where the flat geometry imposes substantially greater challenges than those encountered with a curved faceplate.
The present invention represents an improvement over the various aforementioned prior art approaches by providing a dynamic quadrupole lens for an inline color CRT which applies a sweep-synchronized dynamic quadrupole field to the electron beams to compensate for the astigmatism caused by the beam deflection yoke. The quadrupole field is uniquely defined by the shape of the aligned apertures in the spaced, charged electrodes of the lens as well as by the relative polarity of adjacent electrodes to provide the electron beam convergence or divergence required for beam astigmatism compensation and multi-beam focusing correction.